abstract

7YSZ (yttria stabilized zirconia) was co-doped with metal oxides of different valence, ionic radius and mass in order to investigate microstructural and property changes as a result of co-doping. Mechanical alloying process was used to produce the powder blends which were subsequently sintered at 1500°C for 120 hours. The results from SEM, XRD and DSC showed that the microstructures of the co-doped ternary oxides were affected by the amount of oxygen vacancies in the system, the co-dopant cation radius and mass. Increasing the number of oxygen vacancies by the addition of trivalent co-dopant (Yb2 O3 and Sc2 O3 ) as well as the use of larger cations promoted the stabilization of cubic phase. The tetravalent co-dopant (CeO2 ), on the other hand, had the effect of stabilizing tetragonal phase which may transform into monoclinic phase during cooling, depending on the concentration of tetravalent co-dopant and the mass. Smaller cation mass had the effect of reducing the transformation temperature from tetragonal to monoclinic phase. Pentavalent co-dopants (Nb2 O5 and Ta2 O5 ) were found to stabilize the tetragonal phase at high temperature; however, the stability of the tetragonal phase upon cooling was determined by the mass and ionic radius of the co-dopants. Cation clustering was observed during cooling in trivalent oxide co-doped 7YSZ while clustering of trivalent and pentavalent cations in pentavalent co-doped 7YSZ was not detected. Additionally, from the thermal conductivity measurement results, it was found that trivalent oxides exhibited the most significant effect on reducing the thermal conductivity of ternary oxides; this trend was followed by pentavalent co-doping oxides whereas the tetravalent CeO2 co-doped 7YSZ showed marginal effect. A semi-empirical thermal conductivity model was established based on defect cluster model and the predicted room temperature thermal conductivity values were found to be consistent with that measured experimentally. Furthermore, the incorporation of co-dopant oxide in 7YSZ was observed to substantially modify the elastic modulus of the ternary oxides. More specifically, the addition of co-dopant with larger cation radius was found to reduce the elastic modulus of 7YSZ due to the increase in lattice parameter(s).

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